1. Field of the Invention
The present invention relates to an optical transmitter and an optical transmission control method used for optical transmission, and particularly relates to an optical transmitter and an optical transmission control method used in a wavelength division multiplexing optical transmission system.
2. Description of the Related Art
In optical transmission systems that transmit a large capacity of data at a high bit rate by using an optical fiber as a transmission medium, the transmission capacity has been enlarged more and more by employing wavelength division multiplexing (WDM) in recent years. Optical transmitters in such optical transmission systems employ external modulation systems that enable long distance transmission while keeping wavelength variations small. Among them, a Mach-Zehnder optical modulator is an often-used optical modulator.
FIG. 7 shows a configuration of an optical transmitter using a conventional Mach-Zehnder optical modulator. This optical transmitter includes a wavelength variable light source unit 301, a Mach-Zehnder optical modulator 302 and a modulator driver 303.
Among them, the modulator driver 303 converts an input data signal S into a driving signal SD at a driving voltage needed for the Mach-Zehnder optical modulator 302. The Mach-Zehnder optical modulator 302 modulates light L by changing its light intensity by use of the driving signal SD, and outputs a light signal Lm. Here, the light L is inputted from the wavelength variable light source unit 301.
A dependency of light output on the driving voltage in the Mach-Zehnder optical modulator 302 is expressed as a sinusoidal function of the driving voltage (the horizontal axis) relative to the output light intensity level (the vertical axis) as shown in FIG. 8A. A difference between the driving voltages that take the maximum and minimum values of the output light intensity level is referred to as a Vπ voltage. The Vπ voltage indicates a driving signal voltage that corresponds to an extinction ratio.
Here, the extinction ratio of the light modulator is a ratio between the maximum transmitted light intensity to the minimum transmitted light intensity. The larger this ratio, the higher the performance of the light modulator. In the case where the light source unit 301 outputs light of only one wavelength, the Mach-Zehnder optical modulator 302 easily sets the amplitude voltage of the driving signal SD to the Vπ voltage such that the extinction ratio can be maximized in FIG. 8A.
Recently, however, a wavelength variable light source covering the C band (1530 to 1565 nm) and the L band (1565˜1610 nm) has been developed. Thus, one optical transmitter has been made capable of outputting light of wavelengths in the C band and the L band. For this reason, the wavelength dependence of the Vπ voltage in the Mach-Zehnder optical modulator has become problematic.
FIG. 8B shows the wavelength dependences of the Vπ voltages and variations in light output waveforms under the conditions of two wavelengths λ1 and λ2 in the Mach-Zehnder optical modulator. From FIG. 8B, it is understandable that the Mach-Zehnder optical modulator has a modulation characteristic (Vπ voltage) that changes with a change in the wavelength. In this case, even though the driving voltage is set to the optimum value under the condition of a certain constant wavelength, the extinction ratio is deteriorated due to the wavelength dependence of the Vπ voltage when light of a different wavelength is inputted. Moreover, this also leads to a problem of deteriorating the optical transmission waveform due to turns of the optical transmission waveform at the high level and the low level.
As one of methods for solving these problems, a controller has been provided which obtains, for example, information on changes in light wavelength somehow, and controls a modulator driver so that the modulator driver can output the Vπ voltage corresponding to the obtained wavelength information (Japanese Patent Laid-open Application Publication No. 2004-126024).
However, any method for obtaining light information requires an actual wavelength detector except for a method of estimating the light information from the driving voltage of the wavelength variable light source unit. In addition, a correspondence between the wavelength and the Vπ voltage is determined according to a table including the correspondence stored in advance, and accordingly the modulator driver cannot always be controlled optimally under an actual environment.
As described above, the wavelength dependence of the Vπ voltage of the Mach-Zehnder optical modulator had become a serious problem with the widening of bandwidth of a wavelength variable light source. Accordingly, when the light modulator is driven at a constant driving amplitude, light of a certain transmission wavelength may cause a problem of deteriorating the waveform and the extinction ratio of the light signal, and thereby deteriorating transmission quality.